Vijay Posted August 16, 2019 Share Posted August 16, 2019 Started getting some quotes for the room in roof attic trusses for the single storey part of my build. I'm using metal web joists in my house, but it suddenly occurred to me is it pointless trying to use them in an attic truss? Only had 1 quote using web joists and it was over 3k more than the rest using normal timber joists (11 and 14K) Do web joists offer other advantages vs straight timber joists apart from being able to run services through them? I don't think I'll need to worry about that with attic trusses, as obviously I can run services around the outer part of the trusses (outside the habitable part). How do they compare with deflection? Cheers Vijay Link to comment Share on other sites More sharing options...
PeterW Posted August 16, 2019 Share Posted August 16, 2019 They are much lighter - what’s the span ..? I think MBC use metal web for their rafters to get the depth - @JSHarris or @TerryE could confirm. With anything over a 6m span you are probably going to end up with something that needs the joists to be closer together than 600mm anyway due to the load on the spans. This will play havoc with roof windows as you can’t get a fire egress roof window in a 400mm gap... Link to comment Share on other sites More sharing options...
Vijay Posted August 16, 2019 Author Share Posted August 16, 2019 Internal ICF wall is approx 6300, so overall span will be slightly larger than that as they are bob tail attic trusses. I'll be looking through the quotes a bit closer over the weekend, but wanted closer than 600 as I don't want much deflection. I didn't think about the roof windows............. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted August 16, 2019 Share Posted August 16, 2019 4 minutes ago, PeterW said: They are much lighter - what’s the span ..? I think MBC use metal web for their rafters to get the depth - @JSHarris or @TerryE could confirm. With anything over a 6m span you are probably going to end up with something that needs the joists to be closer together than 600mm anyway due to the load on the spans. This will play havoc with roof windows as you can’t get a fire egress roof window in a 400mm gap... Our MBC rafters are 301mm deep I beams, rather than Posijoists, but for the same reason, they are both stiff and light, so well suited to longer spans. Link to comment Share on other sites More sharing options...
Vijay Posted August 16, 2019 Author Share Posted August 16, 2019 (edited) Sorry, I should have made it clearer in my OP, I meant for the floor section of the attic truss. Edited August 16, 2019 by Vijay Link to comment Share on other sites More sharing options...
ToughButterCup Posted August 16, 2019 Share Posted August 16, 2019 36 minutes ago, Vijay said: [...] Do web joists offer other advantages vs straight timber joists apart from being able to run services through them? [...] Yes. One. But is that advantage; low weight, worth it to you? I'm not as fit as I used to be - but even I can handle a four meter POSI entirely on my own: lift it off the wagon, stack it, store it, swing it into position, line it up with it's partners. But of course, my beer gut got in the way didn't it.... ? Link to comment Share on other sites More sharing options...
Vijay Posted August 16, 2019 Author Share Posted August 16, 2019 Don't think weight comes into it as they are part of the attic truss - I definitely can't list one of those!!! lol Link to comment Share on other sites More sharing options...
Mr Punter Posted August 16, 2019 Share Posted August 16, 2019 Leave it to the designer to decide but I would think timber should be simpler, cheaper and easier to connect on the truss press. You can specify the max floor deflection if you have concerns about bounce. Link to comment Share on other sites More sharing options...
Vijay Posted August 16, 2019 Author Share Posted August 16, 2019 Most suppliers don't do posi web joists to bob tail attic trusses, so that was the point of my post, trying to work out if it's worth the extra money for the suppliers who can accommodate web joists in the trusses Link to comment Share on other sites More sharing options...
ProDave Posted August 16, 2019 Share Posted August 16, 2019 Most roof trusses are about 50mm thick. Most posi joists are nearer 100mm thick. I cannot immediately see an easy way to stamp a bottom chord of a posi joist to the rest of the structure of an attic truss that is much thinner. 1 Link to comment Share on other sites More sharing options...
Vijay Posted August 17, 2019 Author Share Posted August 17, 2019 10 hours ago, ProDave said: Most roof trusses are about 50mm thick. Most posi joists are nearer 100mm thick. I cannot immediately see an easy way to stamp a bottom chord of a posi joist to the rest of the structure of an attic truss that is much thinner. Most suppliers don't, only had 1 quote so far who do Link to comment Share on other sites More sharing options...
Oz07 Posted August 17, 2019 Share Posted August 17, 2019 @Vijay no disrespect think you've got the blinkers on with regards to trying to marry your open webs on floor joists and attic trusses. I'd not worry and go for timber. Like you say plenty of room in eaves for services Link to comment Share on other sites More sharing options...
Vijay Posted August 17, 2019 Author Share Posted August 17, 2019 It's not blinkers, just trying to work out what advantages web joists would have in this situation. Only intend on doing this once so might as well get it right Link to comment Share on other sites More sharing options...
TerryE Posted August 18, 2019 Share Posted August 18, 2019 (edited) We have a warm loft with the loft space forming the 2nd floor, so our design parameters are different. But the corrollary to this is that it is worth considering whether you might ever want to convert your loft area into living space later. If you might then it's a lot cheaper and easier to design your loft floor, roof, etc. with this in mind, and construct it to this specification even if you don't fit it out during build. Edited August 18, 2019 by TerryE Link to comment Share on other sites More sharing options...
Vijay Posted August 18, 2019 Author Share Posted August 18, 2019 Hi Terry, hope you're well. All my loft areas are rooms/future living space, so have been designed from the outset as that Link to comment Share on other sites More sharing options...
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